The human spine consists of 24 bones, which are engaged with each other in joint forms and can support the axial load even in a bent condition. A plurality of discs are arranged between bones to function as a buffer.
The spine has a structure of various names according to the anatomical profile, and in general it has a form bent in an “S” letter shape.
Spinal diseases are caused by impact to the spine by repeatedly applied loads, the decrease of spinal motility due to aging, and the degeneration of the spine itself.
At this time, a frequent and representative symptom of a patient suffering from spinal stenosis is a pain caused by pressed nerves which came out to left or right between bones due to the damage to the disc.
A patient with a spine partly damaged as described above cannot perform activities necessary for his personal life under such a condition, and even if the extent of damage is not so severe, the damaged part of the spine may be pressed or touched by other adjacent parts to cause pain.
So in the case of a patient with part of the spine damaged or injured, an artificial aid should be used in the operation for supporting the adjacent portion of the spine so that the damaged part is not pressed or compressed.
At this time, the artificial aid used for supporting the spine consists of pedicle fixation screws, which are inserted into the top and bottom sides of the damaged spine for a support role, and a spinal rod, which is connected through the pedicle fixation screw for an orthotic role.
The pedicle fixation screws that are inserted into the pedicle structure with the best bone density according to spinal motility can be inserted through several segments of pedicle.
The spinal rod which is fixed in the head portion of the pedicle fixation screws has an anatomical curve that a spinal segment has, and is used for connecting the pedicle fixation screws.
Also, the set screw that fixes the spinal rod to the pedicle fixation screws is a fastening element which converts angular motion into linear motion or inversely by the screw fastening method. Namely, the set screw, which is one of mechanical fastening elements, is a screw for motion that receives large axial force with small angular moment.
Meanwhile, the set screw that fixes and fastens the spinal rod to the pedicle fixation screws should prevent the spinal rod from sliding or detaching.
In the conventional spine fixation device, when joining the male screw of the set screw to the female screw of the pedicle fixation screw, the screw thread surface of the male screw of the set screw does not completely contact the screw thread surface of the female screw of the pedicle fixation screw. Because of that, there occurs a gap between the screw thread surface of the male screw of the set screw and the screw thread surface of the female screw of the pedicle fixation screw, so there is a limit to maintenance of self-locking.
Therefore, in the conventional spine fixation device, the fastening force is decreased as the screws are loosened due to the limit of self-locking between the pedicle fixation screw and the set screw when correcting the pedicle. Accordingly, there is a problem that the spine cannot be stably corrected because the movement of the spinal rod increases.